PH 103 Dr. Cecilia Vogel Lecture 23 Copyright John Aurentz

Review Outline  Radioactive Decay   -decay  exponential decay  Exponential decay cont’d  activity & decay rate relate to N  Particles  fundamental matter particles

Recall Exponential Decay  In nuclear decays the number of nuclei of the original isotope left  follows exponential decay: N = # nuclei (not neutron number)  = decay constant (not wavelength)

Decay Rate  The number of nuclei that decay per unit time is called the decay rate  The more nuclei you have the more nuclei are going to decay,  so the larger the decay rate (activity) at any time… including t=0:

Timing Matters  The half-life affects a nuclide’s usefulness  to applications where you need substantial activity.  or applications where you need to see variations in activity.  For times much longer than the half-life  the activity becomes too small to be useful.  For times much shorter than the half-life  the activity does not change much.

Timing Matters  Applications: 1) Medical tests – need a certain level of activity:  Don’t want sample’s activity to become too low before test is done  Also don’t want sample to keep irradiating the patient long after the test is done. 2) carbon and uranium dating – look for changes  14 C has a half-life of 5,730 yr  good for dating biological samples that are thousands of years old.  235 U has a half-life of 704 million years  good for dating geological samples that are millions of years old.

Particles  Atoms are not indivisible  made up of protons, neutrons, and electrons  Are protons, neutrons, and electrons indivisible?  Electrons are indivisible  protons and neutrons are made up of quarks

Matter Particles  What physicists call “matter” particles  are the particles that make up ordinary matter…  and some other particles with similar properties  Not all particles with mass are called “matter particles”  some are grouped in other categories  such as “anti-matter” and “gauge boson” … more on that soon

Matter Particles  There are two types of matter particles:  quarks and leptons  One big difference between quarks and leptons:  Quarks feel the strong force,  as do protons and neutrons  which are made of quarks.  Leptons, like the electron, do not.

Quarks There are six different quarks (symbol is the first letter) up charm top down strange bottom These have charge = +2/3 e These have charge = -1/3 e Only the two lightest (up and down) are present in ordinary matter. p=uud, and n=udd

Leptons There are six different leptons electron (e - ) muon (  - ) tau (  - ) (electron) neutrino ( e ) mu neutrino (  ) tau neutrino (  ) These have charge = -e These have charge = 0 Only the electron is present in ordinary matter.